High tolerance dimensioning is specified in the manufacture of items in many industries. Tolerances to a mil (0.001 inch) or so have been common for many years. More precise tolerances have been more difficult to achieve. The present invention is directed to a device for consistently boring holes at tolerances less than a tenth of a mil (0.0001 inch), an accuracy unachievable by boring previously.
The present invention allows for manufacture of the tool by a new process which results in a more precisely fabricated tool which in turn results in improved accuracy and precision of the tool. To appreciate the significance of the present invention, it is appropriate to consider a prior art boring head.
A prior boring head 100 is shown in FIG. 2. Boring head 100 includes a primary member 102 and a slide 104. Primary member 102 comprises a body 106 and a shaft 108 extending from a first end of the body 106. Boring head 100 includes a differential screw 107 for moving slide 104 with respect to primary member 102. Differential screw 107 has a first set of threads with a first pitch at one end and a second set of threads with a second pitch at the other end. The first set of threads is threaded into end wall 115 of body 106 and the second set of threads is threaded into guide 130 of slide 104. On turning screw 107, slide 104 moves with respect to head 106 a distance equal to the difference of the pitches of the two sets of threads. Thus, cutting bar 109 held by slide 104 may be moved quite accurately with respect to the axis of shaft 102.
A channel 110 is formed in the second end of body 106. Channel 110 is formed as a dovetail 112 which extends between opposite sides 114 and 116 and is further formed to include a partial, secondary portion 118 beneath dovetail 112. Secondary portion 118 extends inwardly from second side 116, but does not extend all the way to first side 114. Consequently, the portion of side 114 beneath dovetail 112 forms an end wall 115 for secondary portion 118. Secondary portion 118 has a cross-sectional shape, as indicated in FIG. 10, comprising a semi-cylindrical bottom 120 with an axis spaced from the bottom of dovetail 112. The sides 122 of secondary portion 118 are perpendicular to the bottom of dovetail 112 and tangent to semi-cylindrical bottom 120.
To fabricate channel 110, a hole is drilled with a flat bottom bit to form the semi-cylindrical bottom 120 and a temporary, semi-cylindrical upper side shown by broken line 124. The hole extends from second side 116 to a distance spaced from first side 114. Next, dovetail 112 is formed. This leaves trangular cross-sectional portions 126 bordered by portions of side 124, the bottom of dovetail 112 and yet to be formed sides 122. Portions 126 must be removed but are very difficult to remove. Typically, an end mill is used; however, the tool must be long and usually bends during the milling process. Walls 122 often end up having taper and often too, the sidewalls of dovetail 112 near the bottom of dovetail 112 are cut into. The result is that the dovetail walls and walls 122 do not provide a close fit with guides 128, 130.
Slide 104 includes a dovetail male guide 128 at one end and a secondary portion male guide 130 at the other end. Differential screw 107 is threaded into end wall 115 on one side and into secondary portion male guide 130 on the other side. Dovetail guide 128 serves as a stabilizing element.
Screw 132 extends between opposite walls of dovetail 112 between guides 128 and 130 and functions to keep body 106 rigid and to compress the walls the channel 110 against guides 128, 130.
During fabrication of head 100, after primary member 102 and slide 104 are machined, both are heat treated to enhance hardness.
A number of problems prevent the prior art boring head 100 from realizing consistent accuracy. As indicated previously, the milling of triangle 126 frequently leads to taper in walls 122. In addition, since heat treating typically results in warpage, the necessity of machining before heat treating requires correcting for any warpage to the degree possible. Hand lapping of various surfaces of both primary member 102 and slide 104 is often required. This precludes interchangeability of parts. Although the hand lapping improves mating between the slide and the head, it does not eliminate regions of slight interference. Consequently, the slide does not move smoothly during final adjustment with respect to the head, but often jumps and skips causing errors in adjustment of 0.0002 to 0.0004 inches. In addition to the nonuniform (tapered and warped) mating sides of channel 110 and guides 128 and 130, the distance between the differential screw and the outermost corners 132 of the dovetail 112 leads to a torque effect and adds to any binding problem, thereby also increasing inconsistency of adjustment and dial readings.
The problem presented by the prior art is how to move the differential screw closer to the outer dovetail corners to reduce torque during adjustment without weakening either the screw or the dovetail sidewalls, both of which are known high stress elements during boring or adjustment. Inherent in the design problem is how to eliminate the taper and warpage occurring in the mating surfaces of the head and slide during fabrication.